This invention relates to improved polyamide-imide compositions, and to methods for improving the environmental resistance of polyamide-imide compositions. More particularly, this invention relates to polyamide-imide compositions that exhibit improved water absorption characteristics and resistance to the detrimental effects of humid environments. Still more particularly, the improved polyamide-imide compositions of this invention exhibit an improvement in retention of mechanical properties at elevated temperatures after exposure to humid environments.
Polyamide-imides are condensation polymers finding use in a wide range of applications such as adhesives, molding compositions, fibers, films, composites, laminates, etc., owing to a desirable combination of properties. Torlon.RTM. polyamide-imides, available from Amoco Performance Products, Inc., are examples of commercial polyamide-imides.
Despite the many desirable properties of polyamide-imides, their utility in certain applications has been limited by moisture sensitivity, leading to loss of mechanical properties on exposure to high temperatures. In addition, many such polymers, when used as molding resins, do not attain a high level of mechanical properties without additional thermal tempering or similar post treatment of the molded article. So-called annealing or post-curing treatments of polyamide-imide or polyamide-amic acid fabricated parts such as are disclosed in commonly assigned U.S. Pat. No. 4,167,620 allow water liberated due to imidization and chain extension reactions as well as absorbed moisture to diffuse out of fabricated parts and may improve retention of mechanical properties. However, since polyamide-imide resins tend to absorb water when exposed to humid environments, these treatments are not sufficient to permit the use of currently available, commercial polyamide-imides in certain demanding applications. It also is known to add certain metal oxides to polyamide-imides to tie up absorbed moisture as well as water liberated during imidization and chain extension reactions to avoid cracking and sacrifices in physical properties. Again, however, this approach does not yield sufficient improvement of presently available materials or prevent subsequent moisture absorption and further reduction in mechanical properties.
Aromatic polyimides typically do not absorb water to as great an extent as polyamide-imides. However, the utility of aromatic polyimides is limited because of their lack of solubility. Further, their high glass transition temperatures ("Tg") make melt processing impractical or impossible.
U.S. Pat. No. 4,017,459, assigned to the Upjohn Company, discloses amide-imide polymers and copolymers prepared from 2,2-bis(4-(p-aminophenoxy)phenyl) propane and trimellitic anhydride halide or from 2,2-bis(4-(p-isocyanatophenoxy)phenyl) propane and trimellitic acid or anhydride. According to the patent, such polyamide-imides are melt processable, such as by injection molding, and useful in manufacture of articles such as gears, ratchets, clutch linings, bearings, pistons and cams and electrical components. In contrast, the patentee teaches that polyamides prepared from the above-named diamine and isophthalic acid and polyimides prepared from that diamine and pyromellitic acid dianhydride or benzophenone tetracarboxylic acid dianhydride are intractable in the sense of lacking sufficient solubility for solution processing, lacking in melt processability or lacking both.
U.S. Pat. Nos. 4,111,906 and 4,203,922, both assigned to TRW, Inc., disclose that although processability of polyimides can be improved by using the same in predominantly polyamide-amic acid form and imidizing during a final fabrication step, such an approach is disadvantageous because voids in the final products result from water liberated due to the imidization reaction. These patents also state that chemical and thermal stability are improved by preparing polyimides from 2,2-bis(4-(p-aminophenoxy)phenyl) hexafluoropropane. According to the '906 patent, polyimides prepared from this diamine and a dianhydride are useful as coatings, adhesives and as a matrix for laminated glass or graphite structures. Polyimide foams prepared from pyromellitic acid dianhydride or other aromatic tetracarboxylic acid dianhydrides and such diamine in combination with a second aromatic diamine are disclosed in U.S. Pat. No. 4,535,101, assigned to Imi-Tech Corporation. Preparation of polyamides from the above-named diamine and diacids also is disclosed in the '906 patent. The abstracts of both the '906 and '922 patents mention polyamide-imides; however, no additional information is provided.
U.S. Pat. No. 4,340,697, assigned to Toray Industries, Inc., discloses melt processing difficulties with polyamide-imides and purports to remedy the same by blending with polyphenylene sulfide, polyamide, aromatic polyester, polyphenylene ether or a phenoxy resin. According to this patent, polyamide-imides may contain, in addition to a repeating, main structural amide-imide unit, up to 50 mole percent amide or imide units, the latter being introduced into the polymer by replacing a portion of the aromatic tricarboxylic acid component with pyromellitic acid dianhydride or benzophenone tetracarboxylic acid dianhydride.
U.S. Pat. No. 4,599,383, assigned to NTN-Rulon Industries Co., Ltd., discloses compositions having improved water absorption properties containing a polyamide-imide resin in combination with a polyetherimide and a fluoro resin component.
U.S. Pat. No. 4,755,585, assigned to M & T Chemicals, Inc., discloses polyimides, polyamide acids, polyamide-imides, polyesterimides and polyesteramides containing at least 10 mole percent of a reaction product of an aromatic or aliphatic mono- or dianhydride and certain aromatic diamines having an unsubstituted or halogen- or hydrocarbyl-substituted, para-phenylene radical linked by like or different alkylene, alkenylene, sulfide or oxy groups to two unsubstituted or halogen- or hydrocarbyl-substituted, monovalent, aminophenyl radicals provided that the linking groups are not contemporaneously both sulfide or oxy. Such products are said to exhibit improved processing characteristics and thermal stability and to have utility in widespread applications. Two polyamide-imides and films thereof are demonstrated in the examples. Numerous anhydrides, dianhydrides and diamines are named in this publication and mixtures of anhydrides and dianhydrides are mentioned. It is also reported that mixtures of the above-described diamines with other diamines may be used. Interestingly, such other diamines are said to include 2,2-bis(4-(p-aminophenoxy)phenyl) propane and the corresponding sulfone, although the publication also mentions, with supporting citations, that polyimides prepared from such diamines and dianhydrides are insoluble and that polyamide-imides prepared from such diamines are of uncertain solubility and processability.
As shown by the patents discussed above, a great many polyamide-imides and polyimides are known. Although a considerable effort has been expended to develop improved polyamide-imides, most of the presently available resins lack the combination of good processability and excellent as-molded properties with the reduced moisture sensitivity needed for producing composite materials for use in demanding environments. Composite materials and filled molding compounds based on such improved resins would be useful in a variety of applications, including sporting goods and under-the-hood automotive applications, as well as for producing structural components for sports equipment, automobiles, buildings and aircraft.